1. Field of the Invention
The present invention relates to modulation and demodulation apparatuses and methods which are capable of guaranteeing high transmission power efficiency to wired/wireless optical communication apparatuses with an orthogonal frequency division modulation (OFDM) method and an intensity modulation (IM)/direct detection (DD) method.
2. Description of the Related Art
Traditionally, in optical telecommunications fields, where it is very difficult to realize coherent communication systems, IM/DD methods have been widely used. Drivers, light sources, modulators, light detectors, and preamplifiers for the IM/DD methods have been developed and commercialized, thereby enabling the establishment of highly economical systems.
IM methods may be classified into direct modulation methods and indirect modulation methods. Direct modulation methods in which an optical signal is directly modulated by applying a current signal to a light-emitting diode (LED) or a laser diode (LD), and indirect modulation methods in which an optical signal is indirectly modulated by applying a current signal to an electroabsorption modulator (EAM) that is coupled to an LED or an LD. Due to high performance and high cost-effectiveness, the direct modulation methods have been deemed suitable for optical wireless communication systems. Direct modulation methods may be classified into time division (TD) modulation methods such as a non-return-to-zero (NRZ) or return-to-zero on-off keying (OOK) method or a pulse position modulation (PPM) method in which a transmission signal is modulated in consideration of signal intensity or a period between pulses, and frequency division (FD) modulation methods in which a transmission signal is modulated on one or more sub-carriers.
The OOK method, which can be referred to T. S. Chu and M. J. Gans, “High speed infrared local wireless communication,” IEEE Communication Magazine, vol. 25, No. 8, pp. 4-10, August 1987, and the PPM method, which can be referred to D. S. Shiu and J. M. Kahn, “Differential Pulse-Position Modulation for Power-Efficient Optical Communication,” IEEE International Conference on Communication, Montreal, Quebec, Canada, Jun. 8-12, 1997, are highly efficient in terms of using optical signal power and are easy to realize. Thus, the OOK method and the PPM method have been authorized as standard PHY optical modulation/demodulation methods by the Infrared Data Association (IrDA).
However, in an indoor environment where multiple paths may exist, TD modulation methods cannot guarantee a communication speed of 10 Mb/s or higher without the aid of signal interference compensation equipment such as equalizers. In order to address this problem, a turbo equalizer-based method, which can be referred to Michael Tuchler, Ralf Koetter, and Andrew C. Singer, “Turbo Equalization: Principles and New Results,” IEEE Tran. on Comm., vol. 50, No. 5, pp. 754˜767, May 2002, has been developed. The turbo equalizer-based method is a TD modulation-based method of avoiding signal interference without deteriorating the signal-to-noise ratio (SNR) of received signals. However, the turbo equalizer-based method requires a considerable amount of computation, and is difficult and complicated to realize.
The FD modulation method may be classified as a single sub-carrier modulation (SSM) method in which a single sub-carrier is used or a multiple sub-carrier method (MSM) in which a plurality of sub-carriers are used according to the number of sub-carriers used to modulate a transmission signal, which can be referred to J. B. Carruthers and J. M. Kahn, “Multiple-Subcarrier Modulation for Non-Directed Wireless Infrared Communication,” IEEE J. Select. Areas in Commun., vol. 14, pp. 538-546, April 1996.
The MSM OFDM method easy to be realized and easily managing frequency spectrum has been widely used for various wired/wireless communication such as xDSL, wireless LANs, and wireless Internet networks, which can be referred to S. B. Weinstein and P. M. Ebert, “Data Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transform,” IEEE Trans. Commun. Technol., vol. COM-19, pp. 628-634, October 1971. In OFDM, frequency distance between two adjacent subcarriers is equal to the inverse of the period of an OFDM symbol in time. A variety of sub-carriers modulation methods, such as M-ary amplitude shift keying (ASK), M-ary phase shift keying (PSK), M-ary frequency shift keying (FSK), and M-ary quadrature amplitude shift keying (QAM), can be used in OFDM.
However, the OFDM method is likely to lower the operating efficiency of power amplifiers because the peak-to-average power ratio (PAPR) of output waveforms is extremely high. In addition, the OFDM method is likely to deteriorate the performance of communication because of a high possibility of signal distortion due to non-linearity of power amplifiers.
A direct current (DC) bias needs to be applied to a baseband OFDM signals because of the requirement that an IM input signals must be unipolar. The DC bias levels are determined according to a minimum peak level among an IM input signals.
If the average of an IM input signals is 0, the power of the DC bias is proportional to the average power of IM output signals, and the average power of transmission signals is proportional to the average of the absolute values of the IM input signals.
Therefore, assuming that transmission power efficiency is defined as the ratio of the average power of transmission signals to the average power of IM output signals, the transmission power efficiency is equal to the inverse of the PAPR of the input signals to be intensity-modulated. In other words, according to conventional IM methods, IM output signals with a high PAPR can be interpreted as very low signal transmission power efficiency.
A table of FIG. 1, which is an excerpt from Joseph M. Kahn, and John R. Barry, “Wireless Infrared Communications”, IEEE Proceeding, vol. 85, no. 2 pp. 265-298, February 1997, and D. S. Shiu, and J. M. Kahn, “Differential Pulse-Position Modulation for Power-Efficient Optical Communication,” IEEE International Conference on Communication, Montreal, Quebec, Canada, Jun. 8-12, 1997, presents the required average powers and bandwidths of the various modulation schemes compare to the NRZ-OOK.
As indicated in FIG. 1, in an additive white Gaussian noise (AWGN) channel environment where no multiple path interference exists, PPM methods, which are a type of time division method, are efficient in terms of average powers required. The amount of power required by a conventional PSK method such as 2-ary PSK (BPSK) or 4-ary PSK (QPSK) where multiple sub-carriers are used is proportional to the number of sub-carriers used therein.
Therefore, MSM and OFDM method using a considerable number of sub-carriers may not be suitable for IM-based optical signal modulation, even though it has excellent multiple path characteristics.
The present invention provides a technique for addressing the problem of high average power in conventional MSM methods.
Conventional techniques for reducing the amount of power required by MSM and OFDM methods are disclosed in R. YOU and J. M. Kahn, “Average Power Reduction Techniques for Multiple-Subcarrier Intensity-Modulated Optical Signals,” IEEE Trans. on Communication, vol. 49, no. 12, pp. 2164-2171, December 2001, and Shota Teramoto and Tomoaki Ohtsuki, “Multiple-Subcarrier Optical Communication Systems with Peak Reduction Carriers”, IEEE GLOBECOM-2003 Proceeding, pp. 3274-3278, 2003.
These conventional power reduction techniques are identical in that, of a total of N sub-carriers, (N-L) sub-carriers are allocated to information signals, and L sub-carriers are assigned to maximize a minimum peak (a highest negative waveform value) in an OFDM symbol by applying optimization signals, thereby minimizing a DC bias applied during IM, i.e., the average transmission power.
The conventional power reduction techniques may be classified into a method of fixing in advance the locations of L sub-carriers to minimize transmission power and a method of determining the optimum locations of sub-carriers, instead of fixing the locations of the sub-carriers in advance, such that a minimum signal peak in a OFDM symbol can be maximized.
Also, the conventional power reduction techniques may be classified into a method of altering only the phase of signals of the L sub-carriers used to reduce a DC bias and a method of altering both the phase and amplitude, and also, the conventional power reduction techniques may be classified into a fixed bias method in which a fixed DC bias is applied to all OFDM symbols and a time-variable bias method in which the minimum DC bias is applied per each OFDM symbol, and also, conventional OFDM PAPR minimization methods can be classified according to how they optimize waveforms, which can be referred to Seung Hee Han, Jae Hong Lee, “An Overview of Peak-to-Average Power Ratio Reduction Techniques for Multicarrier Transmission”, IEEE Wireless Communications, pp. 56-65, April 2005.
In another conventional PAPR reduction method, which is disclosed in Korean Patent Publication No. 2003-0059523 entitled, “Method and Apparatus for PAPR Reduction Using Soft-Clipping Method in OFDM Wireless Communication System,” a pre-emphasis circuit whose amplification varies in inverse proportion to the amplitude of an MSM or OFDM signal, thereby repressing a signal waveform peak applied to an output power amplifier. The inverse operation of an operation performed at a transmission end is performed at a reception end.
Of the aforementioned conventional methods, the PAPR reduction method, which uses sub-carriers, needs a block coder which converts a signal array comprised of K signals (where K=N−L) into a signal array comprised of N signals.
However, when N is too large, it is very difficult to realize a large-scale block coder which is capable of operating in real time to minimize PAPR.
The aforementioned all disclosed methods to minimize PAPR still have a problem in that the waste of a DC bias power cannot be completely prevented because of the bipolar features of OFDM signals.